There are various current art methods used to designate reserved parking spaces. The most common techniques are painting reserving signs either on a wall of an adjacent building, or on the surface of the parking space itself.
The obvious limitations to these methods are if there is no cooperative building in an appropriate position, or if a potential unauthorized user does not see or simply ignores a space designation.
Another method of assuring that a space is available is to physically block the space. This is generally done by simply placing an obstacle in the space, such as a barrel or a post. However, depending on the installation, these barriers can be moved by determined violators. Making the barriers immovable, such as by locking them down, makes them difficult to deploy. Theses types of barriers also generally require significant storage space as well.